linux-kernel - Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

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Message-ID: <20170731073647.GA26199@bbox>
Date:   Mon, 31 Jul 2017 16:36:47 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "karam . lee" <karam.lee@....com>,
        Jerome Marchand <jmarchan@...hat.com>,
        Nitin Gupta <ngupta@...are.org>, seungho1.park@....com,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>,
        Vishal Verma <vishal.l.verma@...el.com>,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

On Mon, Jul 31, 2017 at 09:17:07AM +0200, Christoph Hellwig wrote:
> On Mon, Jul 31, 2017 at 07:16:59AM +0900, Minchan Kim wrote:
> > rw_page's gain is reducing of dynamic allocation in swap path
> > as well as performance gain thorugh avoiding bio allocation.
> > And it would be important in memory pressure situation.
> 
> There is no need for any dynamic allocation when using the bio
> path.  Take a look at __blkdev_direct_IO_simple for an example
> that doesn't do any allocations.

Do you suggest define something special flag(e.g., SWP_INMEMORY)
for in-memory swap to swap_info_struct when swapon time manually
or from bdi_queue_someting automatically?
And depending the flag of swap_info_struct, use the onstack bio
instead of dynamic allocation if the swap device is in-memory?